1. Field of the Invention
The present invention relates to a record medium and apparatus useful in high density storage of information by optical write and/or read (hereinafter referred to as write/read) methods, and more particularly to improved configurations of optical disk units for use with optical disk write/read apparatus.
2. Description of the Prior Art
Commonly-assigned U.S. Pat. No. 4,365,258, issued Dec. 21, 1982 to F. F. Geyer and E. M. Leonard discloses several optical disk units adapted for high density storage of information. One disclosed configuration comprises (i) a flexible, disk-shaped support web carrying a record layer; (ii) a transparent disk cover sheet opposing the record layer, and (iii) an annular retaining ring for holding the support web and cover sheet, collectively referred to as the web assembly, in a relatively low circumferentially-symmetric tension and tensioned to an operating tension when drawn over a rotatable annular locating surface on the disk unit receiving structure of the optical disk write/read apparatus. Thus the disk unit can normally be kept with the web assembly in low "storage" tension, and used with the web assembly in higher "operating" tension to enhance flatness.
Preferred tensions for the disk-shaped support and cover sheet materials are from substantially zero to just below the elastic limit, or yield point, of those materials. More specifically, the preferred tension depends upon the desired degree of planarity for the particular member (i.e. size, composition, etc.) used. It is preferred that "storage" and "operating" support material tensions be below the elastic limit of the particular material; however, in certain applications some yield can be acceptable as long as surface planarity remains in the desired tolerance. In general, the tension (particularly storage tension) should be selected with respect to the support material so that the stressed material's continuous relaxation over time (i.e. material creep) is slow enough to ensure adequate spacing and planarizing tension throughout the expected product life period.
The above-described disk unit configurations perform admirably. However, it is desirable in some applications to increase the difference between the "storage" tension and the "operating" tension beyond that obtainable by drawing the web assembly over an annular locating surface. Some web materials exhibit objectionably shortened expected product life when subjected to normal "storage" tensions, due to the material's continuous relaxation over time. Yet "storage" tension cannot be decreased without adversely affecting surface planarity at "operating" tension, because the write/read apparatus is capable of increasing tensions by only a predetermined amount.